What is Art and Who Gets to Decide Which Art is Great Art?

While having a discussion with a fellow artist on the website Quora Quora he accused me of in his exact words "using a little piece of rhetoric in which you have changed the label (of the word art) to "anything I or anybody else wants it to be".With this sort of fallacy you can't be proven wrong because any nonsense you might come up with will nicely fit that label.  it's a linguistic trick."
Now this made me think what is the definition of art?
Is art the "Mona Lisa" Mona Lisa?
Is art "Women Three" Woman Three?
Is art "Balloon Dog" Balloon Dog?
Each of these three pieces are very wildly divergent in appearance yet each are worth in the hundreds of millions of dollars.
So where is the trick?
If each and everyone of these three is classified as some of the world's most expensive art who gets to decide what is, or what is not art?
Even more important who gets to decide what is great art?
 Who thinks they are qualified to decide?.
J K Rowling J K Rowling the literary artist of Harry Potter fame, had her manuscript rejected by the first 30 publishers she submitted to and with the 31st publisher who saw it's worth and decided to publish it, became the richest author in the world, her net worth now over a billion dollars.
So ruminating on the subject  I think the only people who can decide if art is art, are the people that buy it or want to buy it.
And that the market sets it own price accordingly.
And yes I will answer to my contentious fellow artist "That is exactly what art is, anything anyone wants it t be."

Truth - Mitro

Balloon Dog - Koons

Mona Lisa - da Vinci

Woman Three -  de Kooning